在“机械能”的教学中,经常碰到车(如汽车、火车)运动时与功率有关的题目,这些题目可分为两类: (一)以额定功率起动,并一直保持不变。额定功率是动力机械正常工作时允许发出的最大功率,超过这个功率机械可能熄火,损坏或烧毁。下面我们分析在额定功率P不变的条件下,当车受到的阻力f也不变时的运动过程:(1)随速度v增加,牵引力F=P/v减小。(2)加速度a(F-f)/m减小。(3)当a减小到为零时,即F=f时,v达到最 In the teaching of “mechanical energy”, there are often problems related to the power of cars (such as cars and trains). These problems can be divided into two categories: (1) Starting with the rated power and keeping it constant. The rated power is the maximum power that is allowed to be issued when the power machine works normally. If it exceeds this power, the machine may be stalled, damaged or burned. In the following, we analyze the motion process when the vehicle’s resistance f is constant under the condition that the rated power P is constant: (1) As the speed v increases, the traction force F=P/v decreases. (2) The acceleration a(F-f)/m decreases. (3) When a decreases to zero, that is, F=f, v reaches the maximum